The 26-year-old was arrested last year after federal employees posing as al-Qaida members delivered materials he requested, including grenades, machine guns and plastic explosives.

Under a plea agreement, federal prosecutors agreed to drop four other charges. Prosecutors and Ferdaus’ lawyers also agreed to request a 17-year sentence on charges that carry a combined maximum of 35 years in prison.

Ferdaus grew up in Massachusetts and has a physics degree from Boston’s Northeastern University.

His mother sobbed uncontrollably after Ferdaus was led away. She had to be helped by Ferdaus’ brother.

Authorities said the explosives were always under the control of federal agents, and the public was never in danger. Counterterrorism experts and model-aircraft enthusiasts say it would be nearly impossible to inflict large-scale damage using model planes.

Prosecutors have said Ferdaus began planning jihad, or holy war, against the United States in early 2010 after becoming convinced through jihadi websites and videos that America was evil. He later contacted a federal informant and began meeting to discuss the plot with undercover agents he believed were members of al-Qaida.

Ferdaus was accused of planning to use three remote-controlled planes measuring 60 to 80 inches in length and capable of speeds greater than 100 mph. Each plane, guided by GPS, was to be packed with 5 pounds of explosives.

In court documents, authorities said Ferdaus traveled to Washington to do surveillance and rented storage space to work on the planes in Framingham.

“I just can’t stop. There is no other choice for me,” according to a recorded conversation detailed in an affidavit filed in court.

Ferdaus’ lawyers had suggested that the FBI ignored signs of mental illness in Ferdaus while investigating him.

During a bail hearing, an FBI agent acknowledged that the FBI had received reports about bizarre behavior by Ferdaus, including a report to Hopkinton police about one incident in which authorities say stood in the road not moving and appeared to have wet his pants.